1. Field of the Invention
This invention is a process for imparting durable flame retardance to cellulosic textile materials. More particularly, it is a process for imparting durable flame retardance to cotton fabric by impregnating the fabric with a condensation product of tetrakis (hydroxy-methyl)phosphonium sulfate and urea and treating the impregnated fabric with a thickened solution of ammonium hydroxide.
2. Description of the Prior Art
U.S. Pat. No. 2,772,188 discloses a process whereby a cellulosic textile material is impregnated with an aqueous solution of a polymerizable methylol-phosphorus polymeric material, containing at least one free methylol group attached to a phosphorus atom; drying the impregnated cellulosic material; and insolubilizing the polymer in situ by contacting it with ammonia. The ammonia may be in the form of gaseous ammonia, ammonium hydroxide, a solution of ammonia in an inert solvent, or as ammonia released in situ by the reaction of a compound capable of releasing ammonia, e.g., an ammonium salt of a weak acid, such as ammonium acetate, or ammonium carbonate.
Using an aqueous ammonia system, such as ammonium hydroxide, or a solution of ammonium carbonate, further polymerization is very rapid and is complete in a few minutes at room temperature, since the ammonia readily penetrates the cellulosic material and rapid curing occurs. Moreover, the aqueous medium absorbs the heat generated by the crosslinking reaction of the ammonia and the polymer. However, in such systems there is a tendency for the polymerizable methylol-phosphorus polymeric material to migrate to the surface of the material before having been immobilized by reaction with the ammonia. This is especially the case if the impregnated cellulose material has not been properly dried.
U.S. Pat. No. 2,983,623 discloses a process wherein a polymerizable methylol phosphorus polymeric material, containing at least one free methylol group attached to a phosphorus atom, is impregnated onto a cellulosic material; the cellulosic material is thoroughly dried, treated with gaseous ammonia, and then treated with aqueous ammonia. However, this process requires thorough drying of the cellulosic material before exposure to the gaseous ammonia and a secondary treatment with aqueous ammonia.
In both of the above-mentioned patents the polymerizable methylol phosphorus polymeric material may be a condensation product formed by reacting tetrakis(hydroxymethyl)phosphonium chloride, or tris(hydroxymethyl)phosphine oxide, or mixtures thereof, with urea.
The process of this invention differs from the processes of U.S. Pat. Nos. 2,772,188 and 2,983,623 (1) in the composition of the condensation product used, (2) in that the moisture content of the dried material is not critical, and (3) in that the ammoniation is carried out in thickened ammonium hydroxide.
In copending, commonly assigned, U.S. patent application, Ser. No. 665,014, now abandoned filed Mar. 8, 1976, a process is disclosed for imparting flame retardancy to cellulosic textile materials by applying thereto novel condensation products of tetrakis(hydroxymethyl)phosphonium sulfate and urea; partially drying the treated textile material, and exposing the partially-dried textile material to gaseous ammonia to effect an insolubilization of the condensation product thereon.
The process of this invention differs from that process in that the treated fabric is not partially dried to a critical moisture content, and in that the condensation product is not insolubilized with gaseous ammonia.
The process of this invention offers the following advantages over the aforementioned processes:
1. It is not necessary to dry the impregnated cellulosic textile material to a critical moisture content.
2. The thickened ammonium hydroxide gives better contact than gaseous ammonia and hence the process gives more uniform results.
3. The increased viscosity of the ammonium hydroxide prevents, or minimizes, the migration of the polymerizable condensation product of tetrakis(hydroxymethyl)phosphonium sulfate and urea to the surface of the cellulosic material before it is immobilized by reaction with ammonia
4. There is no need for a treatment with gaseous ammonia before the treatment with the thickened ammonium hydroxide.
5. The use of the tetrakis(hydroxymethyl)phosphonium sulfate avoids the possibility of formation of bis(chloromethyl)ether, which is very toxic. (See Collier, Environmental Science and Technology, Vol. 6, No. 10, page 930, 1972).